Restructure wrsrvd to support two backends: a headless PixmanRenderer
(default) for running without a display server, and the existing Winit
backend (via --backend winit). The render logic is split into per-backend
modules, and the old render.rs is removed.
Encoder: xor_diff + encode_region (per damage rectangle).
Decoder: apply_region (decompress + XOR apply to framebuffer).
Both live in wayray-protocol::encoding for shared access.
14 tests including 3 end-to-end round-trip integration tests.
Message types for control, display, and input channels with serde
derives. Length-prefixed postcard codec with encode/decode/framing.
Seven round-trip tests covering all message types and edge cases.
